March 9, 2005
Dolores Beasley
Headquarters, Washington
(Phone: 202/358-1753)
Donna Weaver
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4493)
RELEASE: 05-071
NASA'S HUBBLE WEIGHS IN ON THE HEAVIEST STARS IN THE GALAXY
Astronomers have taken an important step toward establishing an upper
limit to the masses of stars. Using NASA's Hubble Space Telescope,
they made the first direct measurement within our Milky Way Galaxy,
and concluded stars cannot get any larger than about 150 times the
mass of our sun.
The astronomers used the Hubble to probe the Arches cluster, the
densest in our galaxy. The finding takes astronomers closer to
understanding the complex star formation process. It also gives the
strongest backing yet to the notion stars have a weight limit.
"This is an incredible cluster that contains a rich collection of some
of the most massive stars in the galaxy, yet it appears to be missing
stars more massive than 150 times the mass of our sun," said
astronomer Donald Figer of the Space Telescope Science Institute,
Baltimore. "Theories predict the more massive the cluster, the more
massive the stars within it. We looked at one of the most massive
clusters in our galaxy and found there is a sharp cutoff to how large
a star can form," he added.
A star's weight ranges from less than one-tenth to more than 100 times
the mass of our sun. Although astronomers know stars come in a
variety of masses, they don't know if the bodies have a weight limit
at birth. Knowing how large a star can form may offer important clues
to how the universe makes them.
Astronomers have been uncertain about how large a star can get before
it cannot hold itself together and blows apart. Astronomers don't
know enough about the details of the star-formation process to
estimate a star's upper mass. Consequently, theories have predicted
stars can be anywhere between 100 to 1,000 times more massive than
the sun.
Figer's finding is consistent with statistical studies of smaller-mass
star clusters in our galaxy and with observations of a massive star
cluster known as R136 in our galactic neighbor, the Large Magellanic
Cloud.
Figer used Hubble's Near Infrared Camera and Multi-Object Spectrometer
to study hundreds of stars ranging from six to 130 solar masses.
Although Figer did not find any stars larger than 130 solar masses,
he conservatively set the upper limit at 150 solar masses. The Arches
cluster is a youngster about 2 to 2.5 million years old. It resides
25,000 light-years away from Earth in our galaxy's hub, a hotbed of
massive star formation. In this region huge clouds of gas collide to
form behemoth stars.
Hubble's infrared camera is well suited to analyze the cluster,
because it penetrates the dusty core of our galaxy. It produces sharp
images, allowing the telescope to see individual stars in a tightly
packed cluster.
Figer estimated the stars' masses by measuring the ages of the cluster
and the brightness of the individual stars. He also collaborated with
Francisco Najarro of the Instituto de Estructura de la Materia in
Madrid, Spain. Najarro produced detailed models to confirm the
masses, chemical abundances and ages of the Arches cluster stars.
"Standard theories predict 20 to 30 stars with masses between 130 and
1,000 solar masses," Figer explained. "But we found none. If they had
formed, we would have seen them," he added.
Figer cautions the upper limit does not rule out the existence of
stars larger than 150 solar masses. His next step is to pinpoint more
clusters to test his weight limit. Several telescopes, including
NASA's Spitzer Space Telescope, have been searching for new star
clusters in the Milky Way.
The finding is published in the March 10 issue of Nature. For imagery
and more information about the research on the Internet, visit:
http://hubblesite.org/news/2005/05 For information about NASA and
agency programs on the Internet, visit: http://www.nasa.gov
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